Electrical circuits requiring high power handling capability while operating at high frequencies such as radio frequencies, S-band and X-band have in recent years become more prevalent. Because of the increase in high power, high frequency circuits there has been a corresponding increase in demand for transistors that are capable of reliably operating at radio frequencies and above while still being capable of handling higher power loads.
Metal-semiconductor field effect transistors (MESFETs) have been developed for high frequency applications. The MESFET construction may be preferable for high frequency applications because only majority carriers carry current. The MESFET design may be preferred over current MOSFET designs because the reduced gate capacitance permits faster switching times of the gate input. Therefore, although all field-effect transistors utilize only majority carriers to carry current, the Schottky gate structure of the MESFET may make the MESFET more desirable for high frequency applications.
As discussed in Split-gate Field-Effect Transistor by Shur (Appl. Phys. Lett., vol. 54, pages 162-164, January 1989) and Dual-Material Gate (DMG) Field Effect Transistor by Long et al. (IEEE Trans. Electron Dev., vol. 46, pp. 865-870, May 1999), performance of field effect transistor (FET) structures may be improved by providing for a lateral variation of the threshold and/or pinch-off voltage along the channel of the device. For example, in a silicon Laterally-diffused metal oxide semiconductor (Si LDMOS) structure the threshold voltage along the channel may be varied using a laterally diffused p-type layer as discussed in Modeling Analysis and Design of RF LDMOS Devices Using Harmonic-Balance Device Simulation by Rotella et al. (IEEE Tran.Electron Dev., vol 48, pp. 991-999, June 2000). As discussed therein, devices having more positive threshold voltages on the source side of the gate exhibit a higher electron velocity over a large part under the gate, higher drain current and higher, more uniform transconductance.